I dont think this is correct in Magmaws case, my guild has managed a total of 34 attempts on Magmaw 10-man and not a single time has anyone who is standing in melee range drawn a Flame Pillar.
It has always targeted the lone Hunter standing at his preset spot 25-30 yards away, except for 2-3 random screwups when a healer or ranged DPS accidentaly stepped out of melee range and thus became an eligible target for the Flame Pillar as well.
I guess its possible we have just been insanely lucky, but looking at the numbers I cant really imagine that being the case.

